Strictly speaking, the theoretical ratio given by Eq. 3 only gives the slope in a flux-variability diagram when monochromatic fluxes are plotted, such as fluxes obtained from spectra after allowance for line emission. If the method is being used with fluxes found from broad-band filter measurements (as is frequently the case), contamination by variable line emission and bound-free emission associated with the BLR causes the unreddened SED to deviate somewhat from the expected theoretical spectrum for a pure externally-illuminated accretion disc (see [1]}). Depending on where variable strong emission lines and bound-free continua fall with respect to filter passbands, the unreddened slope will be more or less than the slope given by Eq. 3. For low-redshift thermal AGNs, only the \(V\)  filter does not include any strong contribution from a broad line. The \(R\)  filter, for example, is centred on the very strong, broad H\(\)  line. This will make the unreddened \(F_V\)  vs. \(F_R\)  gradient much less (i.e., redder) than the value given by Eq.3. The \(R\)  filter, and especially the \(I\)  filter, also include emission from the hot nuclear dust (see [2]} and [3]}). At shorter wavelengths, the \(U\)  filter has a strong contribution from the BLR emission making up the “small blue bump" (see Figure 4 of [1]}). After the \(V\)  band, the filter with the least broad-line emission for low-redshift AGNs is the \(B\)  filter but this includes the higher-order Balmer lines blending into the Balmer continuum emission of the long wavelength side of the small blue bump. The unreddened \(F_B\)  vs. \(F_V\)  flux gradient will thus be greater than the value of 1.07 predicted by Eq. 3. Comparing an observed \(F_B\)  vs. \(F_V\)  gradient with 1.07 therefore gives only a lower limit to \(E(B-V)\) . The effect of the broad-line contamination on the \(B\)  band is illustrated by the distribution of \(F_B\)  vs. \(F_V\)  gradients found by [5]} for a large sample of low-redshift AGNs. Fully 40% of his \(F_B\)  vs. \(F_V\)  gradients are greater than the 1.07 predicted by Eq. 3.
